The present invention generally relates to recording and reproducing apparatus, and more particularly, to apparatus for recording and reproducing television signals using digital techniques.
The video tape recording and reproducing apparatus that is currently most widely used in commercial quality television broadcasting is known as quadruplex format recording apparatus, which has recorded tracks oriented substantially transverse to the longitudinal direction of a magnetic tape. This is accomplished by a rotating wheel typically carrying four equally circumferentially spaced transducing heads which record the television signals on the tape as well as reproduce the same during playback or reproducing. Such commercial grade apparatus record and reproduce FM signals which have attendant, undesirable attributes that continue to exist, in spite of considerable attention that has been focused on them by extremely skilled scientists and engineers. Degradation of the resulting television signal after recording and reproducing is one of the more significant undesirable attributes and it can be due to many causes. Degradation is experienced in the form of Moire, head banding of various types due to mechanical tolerances being exceeded, noise, transients caused by switching of heads and time base errors resulting from changes of tape dimension due to humidity, temperature or servo induced instability and the like. The FM signal is also quite vulnerable to medium surface irregularities, such as scratches, which may be present on the magnetic tape and which affect the signal that is obtained during reproducing. Such recorders are also sensitive to so-called cycle hops and experience degradation during multiple generations of a recording as might occur during an editing process or during reproducing additional copies of a video tape recording. While the reproduced television signal can be applied to a digital time base corrector for the purpose of correcting time base errors, the signal that is obtained off tape which is to be time base corrected nonetheless contains noise, Moire, head switching transients and tape dimension and servo induced errors, all of which can affect the sampling of the analog FM signal for purpose of time base compensation and therefore result in undesirable changes in the subcarrier phase which affect the resulting color and signal timing that is subsequently obtained.
In accordance with the present invention, a television signal in digital form is recorded and reproduced with respect to a magnetic record medium instead of an analog FM type television signal. The television signal is pulse code modulated according to encoding technique that will be hereinafter described to form the digital television signal. The digital recording and reproducing apparatus offers many significantly improved operational characteristics compared to FM recording and reproducing as will be described herein. Among the desirable attributes of the apparatus described herein is the virtual elimination of banding and Moire of any nature from any cause, the reduction of chroma and luminance noise to a value better than -54 db, the ability to relax mechanical tolerances for video head quadrature adjustments by a factor of about 100 and the reduction of inherent time base error to a value that is no greater than about 1/2 nanosecond. Additionally, the apparatus enables perfect color framing to be obtained at all times and introduces virtually no degradation in the television signal during regeneration, which means that essentially unlimited numbers of generations of a video tape recording can be made. Moreover, since the decision for making the phasing selection for digitally sampling the analog color television signal is precisely determined with respect to the location of a newly generated horizontal synchronization pulse before recording, the apparatus is completely immune to cycle hops, which are a problem with present video tape recorders having to make an initializing decision at each start of reproducing operation. Also, tape irregularities, such as scratches and surface roughness which cause drop-outs in FM recording is significantly less consequential to the operation of the system described herein, which means that less expensive video tape can be used to produce a significantly improved quality recording compared to an FM recording.
To effect synchronous reproduction of the recorded digital television signals in apparatus employing rotating magnetic heads to record and reproduce the signals with respect to a magnetic record medium transported past the heads, digital synchronizing information is inserted within the horizontal blanking interval of the television signal coherently with the video data interval of the signal. The synchronizing information is encoded to identify each of the various horizontal lines and fields forming the multiple field sequence of the television signals. Upon reproduction of the digital television signals, the synchronizing information is extracted from the reproduced signal and used to control the speed of rotation of the heads, the speed of transport of the record medium and the rotation position of the head relative to the record medium with respect to a controlling reference identifying the condition for synchronous reproduction of the recorded television signal. This control is exercised to effect synchronous reproduction of the recorded information according to the condition identified by the controlling reference. The use of the inserted synchronizing information in the control of the rotating heads and transported record medium facilitates rapid achievement of the desired synchronous reproduction condition.
Other advantages will become apparent upon reading the following detailed description while referring to the attached drawings.